Nitride semiconductor light-emitting device and optical device including the same

ABSTRACT

A nitride semiconductor light-emitting device includes an emission layer ( 103 ) formed on a substrate ( 100 ), and the emission layer includes a quantum well layer of GaN 1−x-y−z As x P y Sb z  (0&lt;x+y+z≦0.3) containing Al.

TECHNICAL FIELD

[0001] The present invention relates to a nitride semiconductorlight-emitting device having high luminous efficiency and an opticaldevice utilizing the same.

BACKGROUND ART

[0002] In general, a nitride semiconductor is utilized or studied for alight-emitting device or a high-power semiconductor device. In the caseof the nitride semiconductor light-emitting device, a quantum well layeremitting light in an emission layer is made of InGaN containing indium,and light-emitting devices for a wide color range from blue to orangecan be prepared by varying the In content. In recent years, a blue orgreen light-emitting diode or a bluish purple semiconductor laser hasbeen developed utilizing the characteristics of the nitridesemiconductor light-emitting device. Japanese Patent Laying-Open10-270804 reports a light-emitting device including an emission layercontaining a GaNAs well layer/a GaN barrier layer.

[0003] In the case of preparing a light-emitting device utilizing anInGaN quantum well layer, however, the chemical thermal equilibriumstate of the InGaN layer is so unstable that it is difficult to form anemission layer having excellent crystallinity. Particularly in the caseof growing an InGaN crystal layer containing In of at least 15% in thegroup III elements, the InGaN crystal is readily phase-separated intoregions having high and low In contents respectively depending on itsgrowth temperature (this phenomenon is hereinafter referred to also asconcentration separation). Such concentration separation causesreduction of luminous efficiency and increase of the half-width of theemission wavelength (color heterogeneity).

[0004] On the other hand, a GaNAs well layer (As is at least partiallyreplaceable with P and/or Sb) containing no In does not cause theaforementioned problem of concentration separation, but causes phaseseparation containing different crystal systems (separation of ahexagonal system and a cubic system) due to As etc. contained therein,and such crystal system separation causes reduction of crystallinity andluminous efficiency of the well layer.

[0005] Accordingly, a principal object of the present invention is toimprove luminous efficiency of a nitride semiconductor light-emittingdevice including an emission layer containing a quantum well of nitridesemiconductor by improving crystallinity and suppressing phaseseparation of the quantum well layer.

DISCLOSURE OF THE INVENTION

[0006] A nitride semiconductor light-emitting device according to anaspect of the present invention includes an emission layer formed on asubstrate, and this emission layer includes a single quantum well layerof GaN_(1−x-y-z)As_(x)P_(y)Sb_(z) (0<x+y+z≦0.3) containing Al.

[0007] The substrate preferably is formed with a nitride semiconductor,and may also be a pseudo GaN substrate. The etch pit density of thesubstrate corresponding to threading dislocation density is preferablynot more than 7×10⁷/cm².

[0008] The single quantum well layer preferably contains Al inconcentration of at least 6×10¹⁸/cm³, and preferably has a thickness ofat least 0.4 nm and not more than 20 nm. The well layer preferablycontains a dopant of at least any of Si, O, S, C, Ge, Zn, Cd and Mg, andthe dopant concentration is preferably in a range of 1×10¹⁶/cm³ to1×10²⁰/cm³.

[0009] According to another aspect of the present invention, a nitridesemiconductor light-emitting device includes an emission layer having amultiple quantum well structure obtained by alternately stacking aplurality of quantum well layers and a plurality of barrier layers on asubstrate, the quantum well layers is formed withGaN_(1−x-y-z)As_(x)P_(y)Sb_(z) (0≦x≦0.10, 0≦y≦0.16, 0≦z≦0.04, x+y+z>0)and additionally contain at least Al, and the barrier layers formed withnitride semiconductor.

[0010] GaN is preferably employable as the substrate material. Theemission layer causing action of emitting light includes the quantumwell layers and the barrier layers, and the quantum well layers have asmaller energy band gap as compared with the barrier layers.

[0011] The Al content of the well layers is preferably at least1×10¹⁹/cm³. The barrier layers preferably contain any element selectedfrom As, P and Sb.

[0012] The emission layer preferably includes at least 2 and not morethan 10 well layers. The quantum well layers each preferably have athickness of at least 0.4 nm and not more than 20 nm. The barrier layerseach preferably have a thickness of at least 1 nm and not more than 20nm.

[0013] The nitride semiconductor light-emitting device preferablyincludes a substrate, and at least either a first adjacent semiconductorlayer in contact with a first main surface, included in both mainsurfaces of the emission layer, closer to the substrate or a secondadjacent semiconductor layer in contact with a second main surfacefarther from the substrate preferably formed with a nitridesemiconductor containing Al.

[0014] At least any dopant of Si,O, S, C, Ge, Zn, Cd and Mg ispreferably added to at least either the well layers or the barrierlayers. A content of such a dopant is preferably in a range of 1×10¹⁶ to1×10²⁰/cm³.

[0015] The aforementioned nitride semiconductor light-emitting device ispreferably employable in various optical devices such as an opticalinformation reader, an optical information writer, an optical pickup, alaser printer, a projector, a display and a white light source.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a schematic sectional view showing structure of anitride semiconductor diode device according to Embodiment of thepresent invention.

[0017]FIG. 2 is a schematic sectional view showing an exemplary pseudoGaN substrate.

[0018]FIGS. 3A and 3B are schematic sectional views for illustrating aprocess of manufacturing the pseudo GaN substrate.

[0019]FIG. 4 is a schematic sectional view of a light-emitting diodedevice according to another Embodiment.

[0020]FIG. 5 is a top plan view of the light-emitting diode device shownin FIG. 4.

[0021]FIG. 6 is a graph showing influence exerted by Al content in aquantum well layer on the degree of the crystal system separation andthe luminous intensity.

[0022]FIG. 7 is a schematic sectional view showing structure of anitride semiconductor laser device according to still anotherEmbodiment.

[0023]FIG. 8 is a schematic top plan view for illustrating chip divisionof a laser device according to Embodiment.

[0024]FIG. 9 is a graph showing relation between the number of welllayers of laser devices and threshold current densities of the devices.

[0025]FIGS. 10A and 10B schematically illustrate energy band gapstructures in light-emitting devices according to Embodiments.

[0026]FIGS. 11A and 11B schematically illustrate other exemplary energyband gap structures in light-emitting devices according to Embodiments.

[0027]FIG. 12 schematically illustrates a further exemplary energy bandgap structure in a light-emitting device according to Embodiment.

[0028]FIG. 13 is a schematic sectional view showing structure of a laserdevice employing a nitride semiconductor substrate according toEmbodiment.

[0029]FIG. 14 is a schematic sectional view showing a nitridesemiconductor thick-film substrate utilizable in a light-emitting deviceaccording to the present invention.

[0030]FIG. 15A is a schematic sectional view showing an exemplarylight-emitting diode device according to the present invention, and FIG.15B is a schematic top plan view corresponding to the diode device shownin FIG. 15A.

[0031]FIG. 16 is a graph showing relation between the number of welllayers in light-emitting diode devices according to the presentinvention and luminous intensities of the devices.

[0032]FIG. 17 is a schematic block diagram showing an optical diskrecording/reproducing apparatus as an exemplary optical device employinga light-emitting device according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0033] Generally in the case of growing a nitride semiconductor crystallayer, it is possible to use a substrate of GaN, sapphire or the like,and similarly possible to use another nitride semiconductor substratesuch as of Al_(a)Ga_(b)In_(c)N (0≦a≦1, 0≦b≦1, 0≦c≦1, a+b+c=1). Thenitrogen element in this substrate may be replaced with As, P or Sb in arange of not more than about 10% thereof (on condition that thehexagonal system is maintained). In the case of a nitride semiconductorlaser, a couple of layers having a lower refractive index than a coupleof cladding layers sandwiching an emission layer must be in contact withthe outer side of the cladding layers in order to render a verticaltransverse mode unimodal, and then an AlGaN substrate is preferablyemployed. Further, the substrate may be doped with Si, O, Cl, S, C, Ge,Zn, Cd, Mg or Be. Among these doping agents, Si, O and Cl areparticularly preferable for an n-type nitride semiconductor substrate.

[0034] While the following Embodiments are described with reference to aC-plane {0001} substrate of sapphire or a nitride semiconductor, an Aplane {11-20}, an R plane {1-102} or an M plane {1-100} may also beemployed as the plane orientation forming the main surface of thesubstrate in place of the C plane. In the case that a substrate has anoff-angle within 2 degrees from such plane orientation, surfacemorphology of a semiconductor crystal layer grown thereon is improved.

[0035] The crystal layer is generally grown by metal organic chemicalvapor deposition (MOCVD), molecular beam epitaxy (IBE), hydride vaporphase epitaxy (HVPE) or the like.

[0036] The conventional GaNAs well layer disclosed in Japanese PatentLaying-Open 10-270804 contains no In and hence causes no phaseseparation due to In. However, the well layer containing As causescrystal system separation, and thus causes reduction of crystallinityand reduction of luminous efficiency in a finally obtained nitridesemiconductor light-emitting device.

[0037] This crystal system separation possibly occurs not only in theGaNAs well layer but also in a GaNP well layer or a GaNSb well layer.Thus, it is conceivable that crystal system separation of a well layeris caused by containing As, P or Sb.

[0038] This crystal system separation conceivably results from that As,P and Sb are more adherent to Ga as compared with N and that N hasextremely high volatility as compared with As, P and Sb (N escapes fromthe crystal). In a step of supplying a raw material for Ga and a rawmaterial for N for vapor phase epitaxy of a GaN crystal, part of thesupplied N material is partially bonded to the Ga material to form theGaN crystal on the outermost surface (epitaxial growth surface) of theGaN crystal, while most part thereof conceivably re-evaporates due tothe high volatility of N.

[0039] On the other hand, Ga having not incorporated in the GaN crystaldue to the re-evaporation of N momentarily diffuses on the epitaxialgrowth surface and thereafter re-evaporates. If a raw material for As, Por Sb is supplied in addition to the N material, however, the remainingGa readily adsorbs to As, P or Sb while diffusing on the epitaxialgrowth surface. This is because the adherent of As, P or Sb to Ga ismuch higher with respect to N. Thus, bonds of Ga—As, Ga—P or Ga—Sb areconceivably formed with high probability. Further, the surface migrationlength of Ga is so large that there is high probability that the bondsof Ga—As, Ga—P or Ga—Sb encounter each other, and these bonds can befixed and crystallized upon these encounters. Thus, the aforementionedsegregation effect may conceivably occur. If the degree of thissegregation effect increases, it finally causes separation into regions(cubic system) having a high bonding ratio of Ga—As, Ga—P or Ga—Sb andregions (hexagonal system) having a low bonding ratio. This isconceivably the crystal system separation. In order to reduce thiscrystal system separation, therefore, it is important to efficientlyincorporate N into the crystals.

Embodiment 1

[0040] In a nitride semiconductor light-emitting device according toEmbodiment 1 of the present invention, it is possible to reduce crystalsystem separation by introducing Al into a single quantum layer ofGaN_(1−x-y-z)As_(x)P_(y)Sb_(z) (where 0<x+y+z≦0.3) included in anemission layer, as hereinafter described. This is conceivably because Alhaving extremely high reactivity with respect to N as compared with Gaacts to prevent N from escaping from the well layer. Further, thesurface migration length of Al is short as compared with that of Ga andhence the aforementioned remarkable segregation effect is conceivablynot caused even if Al is bonded to As, P or Sb. Thus, it is conceivablypossible to reduce crystal system separation by adding Al into theGaN_(1−x-y-z)As_(x)P_(y)Sb_(z) single well layer (where 0<x+y+z≦0.3).

[0041] (As to Composition Ratio of As, P or Sb in Single Well Layer)

[0042] The total composition ratio x+y+z of As, P and Sb in theAlGaN_(1−x-y-z)As_(x)P_(y)Sb_(z) single well layer in the nitridesemiconductor light-emitting device is preferably set to at least 0.01%and not more than 30%, and more preferably at least 0.1% and not morethan 10%. If the composition ratio x+y+z is smaller than 0.01%, it isdifficult to attain improvement of luminous intensity by introducing As,P or Sb into the single well layer. If the composition ratio x+y+z ishigher than 30%, on the other hand, it is difficult to reduce crystalsystem separation caused by As, P or Sb even if Al is added to thesingle well layer. If the composition ratio x+y+z is at least 0.1% andnot more than 10%, the effect caused by adding Al can be attainedsufficiently.

[0043] (As to Thickness of Single Well Layer)

[0044] While a preferred thickness of the Al_(a)Ga_(1−a)N1-x-y-zAsxPySbz(0<x+y+z≦0.3) single well layer depends on the Al composition ratio a,it is possible to thickly grow the layer to a thickness of about 100 nmin the case of satisfying the average composition ratio (0<x+y+z≦0.3) ofAs, P or Sb. This is conceivably because the crystal system separationis reduced due to the average composition ratio. In consideration of thelight-emitting device, however, an effective thickness of the singlewell layer is preferably in a range of at least 0.4 nm and not more than20 nm. If the thickness of the single well layer is less than 0.4 nm,there is a possibility that a carrier confinement energy level caused bythe quantum well effect becomes so high that the luminous efficiency isreduced. If the thickness of the single well layer exceeds 20 nm, on theother hand, there is a possibility that electric resistance of thedevice is increased.

[0045] (As to Al Content of Single Well Layer)

[0046]FIG. 6 illustrates influence exerted by Al addition into aGaN_(0.92)P_(0.08) single well layer on the degree of crystal systemseparation and luminous intensity. Referring to FIG. 6, the horizontalaxis shows the Al content in the well layer, the left vertical axisshows the degree (%) of crystal system separation and the right verticalaxis shows the luminous intensity. The luminous intensity in FIG. 6 isnormalized with reference to the luminous intensity obtained when no Alis added. The degree of crystal system separation expresses the volumefraction of a portion causing crystal system separation in the unitvolume of the well layer.

[0047] As understood from FIG. 6, the degree (%) of crystal systemseparation starts to decrease when the Al content is increased fromaround 6×10¹⁸/cm³, and reaches a level of not more than 3% when the Alcontent exceeds 1×10¹⁹/cm³. On the other hand, the luminous intensitystarts to increase when the Al content is increased from around6×10¹⁸/cm³, and reaches a level of at least 10 times when the Al contentexceeds 1×10¹⁹/cm³. From these relative facts, there is conceivablycorrelation between the crystal system separation and the luminousintensity.

[0048] From the above, the degree of crystal system separation ispreferably not more than 6%, and more preferably not more than 3%, inorder to obtain a single well layer having high luminous intensity (highluminous efficiency). In order to obtain such a degree of crystal systemseparation, the Al content is preferably at least 6×10¹⁸/cm³, and morepreferably at least 1×10¹⁹/cm³.

[0049] The upper limit of the Al content is preferably not more than 0.2(corresponding to a content of not more than 8.8×10²¹/cm³) whenexpressed by the Al composition ratio a in theAl_(a)Ga_(1−a)N_(1−x-y-z)As_(x)P_(y)Sb_(z) single well layer, and morepreferably not more than 0.1 (corresponding to a content of not morethan 4.4×10²¹/cm³). In this case, the composition ratio of As, P or Sbmust be 0<x+y+z≦0.3. If the composition ratio a of Al exceeds 20%,crystallinity of the single well layer is so reduced that the luminousefficiency is undesirably reduced. If the composition ratio a of Al isnot more than 10%, the operating voltage of the device can be preferablyreduced.

[0050] While FIG. 6 shows the case of adding Al into theGaN_(0.92)P_(0.08) crystal, it is possible to cause a tendency similarto that shown in FIG. 6 also when Al is added intoGaN_(1−x-y-z)As_(x)P_(y)Sb_(z) (0<x+y+z≦0.3) crystals.

[0051] (As to Emission wavelength of Single Well Layer)

[0052] In the Al_(a)Ga_(1−a)N_(1−x-y-z)As_(x)P_(y)Sb_(z) (0<x+y+z≦0.3)single well layer, a target emission wavelength can be obtained bymainly adjusting the composition ratio of As, P or Sb.

[0053] In order to obtain an emission wavelength in the vicinity of 380nm of the ultraviolet when Al is added at a doping level (i.e., the Alcomposition ratio of the single well layer is less than 1%) as shown inFIG. 6, for example, x may be equal to 0.05 in the case ofAlGaN_(1−x)As_(x), y may be equal to 0.01 in the case ofAlGaN_(1−y)P_(y), and z may be equal to 0.02 in the case ofAlGaN_(1−z)Sb_(z). In order to obtain an emission wavelength in thevicinity of 410 nm of bluish-purple, x may be equal to 0.02 in the caseof AlGaN_(1−x)As_(x), y may be equal to 0.03 in the case ofAlGaN_(1−y)P_(y), and z may be equal to 0.01 in the case ofAlGaN_(1−z)Sb_(z). In order to obtain a wavelength in the vicinity of470 nm of blue, further, x may be equal to 0.03 in the case ofAlGaN_(1−x)As_(x), y may be equal to 0.06 in the case ofAlGaN_(1−y)P_(y), and z may be equal to 0.02 in the case ofAlGaN_(1−z)Sb_(z). In order to obtain a wavelength in the vicinity of520 nm of green, still further, x may be equal to 0.05 in the case ofAlGaN_(1−x)As_(x), y may be equal to 0.08 in the case ofAlGaN_(1−y)P_(y), and z may be equal to 0.03 in the case ofAlGaN_(1−z)Sb_(z). In order to obtain a wavelength in the vicinity of650 nm of red, still further, x may be equal to 0.07 in the case ofAlGaN_(1−x)As_(x), y may be equal to 0.12 in the case ofAlGaN_(1−y)P_(y), and z may be equal to 0.04 in the case ofAlGaN_(1−z)Sb_(z). In preparing the single well layer in the vicinity ofthe aforementioned composition ratio, it is possible to substantiallyobtain a target emission wavelength.

[0054] When Al is added at a composition ratio level (i.e., the Alcomposition ratio is at least 1%), the composition ratio of As, P or Sbmay be adjusted to be relatively high. Tables 1 and 2 show specificrelation between the composition ratio of As or P and the emissionwavelength. Table 1 shows the relation between the Al composition ratio(a) and the As composition ratio (x) for obtaining a target emissionwavelength with an Al_(a)Ga_(1−a)N_(1−x)As_(x) single well layer. Table2 shows the relation between the Al composition ratio (a) and the Pcomposition ratio (y) for obtaining a target emission wavelength with anAl_(a)Ga_(1−a)N_(1−y)P_(y) single well layer. The composition ratio ofSb is preferably not more than 0.04. This is because the crystallinityis remarkably reduced if the composition ratio of Sb is higher than0.04. TABLE 1 Al_(a)Ga_(1-a)N_(1-x)As_(x) a = 0.01 a = 0.02 a = 0.03 a =0.05 a = 0.1 a = 0.2 Emis- 380 nm 0.01 0.01 0.01 0.01 0.02 0.03 sion 400nm 0.01 0.02 0.02 0.02 0.02 0.04 Wave- 410 nm 0.02 0.02 0.02 0.02 0.030.04 length 470 nm 0.04 0.04 0.04 0.04 0.05 0.06 520 nm 0.05 0.05 0.050.05 0.06 0.07 650 nm 0.07 0.07 0.07 0.08 0.08 0.10

[0055] TABLE 2 Al_(a)Ga_(1-a)N1-yP_(y) a = 0.01 a = 0.02 a = 0.03 a =0.05 a = 0.1 a = 0.2 Emis- 380 nm 0.01 0.01 0.01 0.02 0.03 0.04 sion 400nm 0.02 0.02 0.03 0.03 0.04 0.06 Wave- 410 nm 0.03 0.03 0.03 0.04 0.040.06 length 470 nm 0.06 0.06 0.06 0.07 0.07 0.10 520 nm 0.08 0.08 0.080.09 0.10 0.12 650 nm 0.12 0.12 0.12 0.13 0.14 0.16

[0056] (As to Substrate for Growing Light-Emitting Device IncludingSingle Well Layer)

[0057] The inventors have found that luminous intensity of alight-emitting device including a single well layer varies with asubstrate for growing the single well layer. This is conceivably becausecrystal defect density in the light-emitting device varies with thesubstrate and the surface migration length of Al is so short that Al isreadily trapped in the vicinity of crystal defects. Consequently, it isconceivable that the effect of reducing crystal system separation byadding Al is attained only in the vicinity of crystal defects and it isimpossible to sufficiently attain the reductive effect on the overallsubstrate.

[0058] According to recognition by the inventors, the luminous intensityis strong when a light-emitting device including a single well layer isgrown on a nitride semiconductor substrate, i.e., the nitridesemiconductor substrate is the most preferable substrate. For example,the etch pit density of a nitride semiconductor film grown on a GaNsubstrate is not more than about 5×10⁷/cm². This is a value smaller thanthe etch pit density (at least about 4×10⁸/cm²) of a nitridesemiconductor film on a sapphire substrate or an SiC substrate(substrate other than the nitride semiconductor substrate) used as thesubstrate for a conventional nitride semiconductor light-emittingdevice. The etch pit density is obtained by dipping an epi-wafer(light-emitting device) for 10 minutes in an etching solution(temperature: 250° C.) containing phosphoric acid and sulfuric acid inthe ratio 1:3 and measuring the density of pits formed on the surface ofthe wafer. In this case, the pit density on the surface of the epi-waferis measured as the etch pit density, and hence no crystal defect densityof the well layer is measured in a strict sense. When the etch pitdensity is high, however, crystal defect density in the well layer isalso increased in proportion thereto, and hence measurement of the etchpit density can be regarded as the index as to whether or not the welllayer contains a large number of crystal defects.

[0059] A substrate preferable subsequently to the nitride semiconductorsubstrate is a pseudo GaN substrate. A method of manufacturing a pseudoGaN substrate is described in detail in Embodiment 2. The etch pitdensity of a nitride semiconductor film grown on the pseudo GaNsubstrate is not more than about 7×10⁷/cm² in a region of the smallestetch pit density. This is a value close to the etch pit density of thenitride semiconductor film grown on the GaN substrate. However, thepseudo GaN substrate, including regions of low etch pit density andregions of high etch pit density in a mixed state, tends to reduce theyield of the light-emitting devices as compared with the GaN substrate(exemplary nitride semiconductor substrate). On the other hand, thepseudo GaN substrate has such an advantage as that a substrate having alarger area than the nitride semiconductor substrate can be manufacturedat a low cost.

[0060] (As to Addition of Impurity to Single Well Layer)

[0061] According to photoluminescence (PL) measurement by the inventors,PL luminous intensity is strengthened to about 1.2 times when Si isadded into the single well layer. In other words, luminous intensity ofthe light-emitting device can be improved by adding an impurity to thesingle well layer. This is conceivably for the following reason. In thesingle well layer according to the present invention, it is possible toeffectively reduce the crystal system separation by adding Al. However,Al is conceivably readily trapped in the vicinity of defects in thecrystal, since Al has a shorter surface migration length on an epitaxialgrowth surface as compared with Ga. Consequently, the effect ofsuppressing crystal system separation can act mainly in the vicinity ofcrystal defects.

[0062] Therefore, the impurity of Si is preferably added into the singlewell layer. The impurity is homogeneously distributed on the overallsurface of the epitaxial growth film, and forms nuclei for crystalgrowth. These nuclei thinkably have action of trapping Al similarly tothe crystal defects. Further, these nuclei are homogeneously distributedon the overall surface of the epitaxial growth film dissimilarly to thecrystal defects, to conceivably cause action of homogeneouslydistributing Al on the overall single well layer. Thus, the effect ofreducing crystal system separation conceivably occurs efficiently toresult in improvement of the luminous intensity. Particularly in alight-emitting device including a single well layer grown on a substratesuch as a sapphire substrate, for example, other than a nitridesemiconductor substrate, the effect resulting from addition of theimpurity is remarkable with a large number of crystal defects (etch pitdensity: at least 4×10⁸/cm²).

[0063] It is possible to attain a similar effect also when an impurityof at least one of O, S, C, Ge, Zn, Cd and Mg is added in place of Si. Acontent of the impurity is preferably in a range of 1×10¹⁶/cm³ to1×10²⁰/cm³. When the content of the impurity is smaller than 1×10¹⁶/cm³,no improvement of the luminous intensity of the light-emitting device isattained. When the content of the impurity is larger than 1×10²⁰/cm³, onthe other hand, the crystallinity is undesirably deteriorated (theluminous efficiency is reduced).

[0064] (Light-Emitting Diode Device Including Single Well Layer)

[0065]FIG. 1 shows an exemplary nitride semiconductor light-emittingdiode device including a single well layer as a schematic sectionalview. This diode device includes an n-type GaN substrate 100 having a Cplane (0001) as the main surface, a GaN buffer layer 101 (thickness: 100nm) formed at a relatively low temperature, an n-type GaN layer 102(thickness: 3 μm, Si impurity concentration: 1×10¹⁸/cm³), a single welllayer 103, a p-type Al_(0.1)Ga_(0.9)N carrier blocking layer 104(thickness: 20 nm, Mg impurity concentration: 6×10¹⁹/cm³), a p-type GaNcontact layer 105 (thickness: 0.1 μm, Mg impurity concentration:1×10²⁰/cm³), an optically transparent electrode 106, a p electrode 107and an n electrode 108.

[0066] In formation of the diode device shown in FIG. 1, the n-type GaNsubstrate 100 is first set in an MOCVD (metal organic chemical vapordeposition) apparatus to grow the GaN buffer layer 101 to a thickness of100 nm at a relatively low substrate temperature of 550° C. with NH₃(ammonia) as a raw material for a group V element and TMGa (trimethylgallium) as a raw material for a group III element. Then, SiH₄ (silane)is added to the NH₃ and TMGa at a substrate temperature of 1050° C. toform the n-type GaN layer 102 (Si impurity concentration: 1×10¹⁸/cm³) toa thickness of 3 μm. Thereafter, the substrate temperature is reduced to800° C., to grow the Al_(0.01)Ga_(0.09)N_(0.092)P_(0.08) single welllayer 103 of 4 nm thickness. At this time, SiH₄ (Si impurityconcentration: 1×10¹⁸/cm³) is added to the single well layer.

[0067] Then, the substrate is heated to 1050° C. again to grow thep-type Al_(0.01)Ga_(0.09)N carrier blocking layer 104 of 20 nm thicknessand the p-type GaN contact layer 105 of 0.1 μm thickness. EtCP₃Mg(bisethylcyclopentadienyl magnesium) is added with an Mg concentrationin a range of 5×10¹⁹/cm³ to 2×10²⁰/cm³ as the p-type impurity. Thep-type impurity concentration in the p-type GaN contact layer 105 ispreferably increased as approaching the surface on which the opticallytransparent electrode 106 is to be formed. This is because contactresistance of the p electrode can thereby be reduced while suppressingincrease of crystal defects resulting from addition of the impurity. Asmall quantity of oxygen may be mixed during growth of the p-type layersin order to remove residual hydrogen acting to hinder activation of Mgin the p-type layers.

[0068] After growing the p-type GaN contact layer 106, gas in thereaction chamber of the MOCVD apparatus is replaced with nitrogen andNH₃, and the substrate temperature is reduced at a rate of 60° C./min.Supply of NH₃ is stopped when the substrate temperature is reduced to800° C., and the substrate is held at this temperature for 5 minutes andthereafter cooled to the room temperature. The temperature for holdingthe substrate is preferably in a range of 650° C. to 900° C., and theholding time is preferably in a range of at least 3 minutes and not morethan 10 minutes. The temperature reduction rate from the holdingtemperature is preferably at least 30° C./min. As a matter of fact,according to a result of Raman measurement, the film grown in the abovemanner already exhibited the p-type property (Mg was activated), eventhough the same was not subjected to the conventional annealing foractivating the p-type impurity. Contact resistance after formation ofthe p electrode (described in detail later) was also reduced, eventhough the conventional annealing for activating the p-type impurity wasnot carried out. Of course, however, if the conventional annealing foractivating the p-type impurity is carried out, the activation ratio ofMg is further improved.

[0069] Then, the epi-wafer is taken out from the MOCVD apparatus, andthe electrodes are formed thereon. In this Embodiment, the n-type GaNsubstrate 100 is employed and hence the n electrode 108 is formed inorder of Hf/Au on the back surface thereof. In place of this n electrodematerial, Ti/Al, Ti/Mo or Hf/Al may also be employed. Particularly whenHf is employed for the n electrode, the contact resistance of theelectrode is preferably reduced. In formation of the p electrode, a Pdfilm having an extremely small thickness of 7 nm is deposited as theoptically transparent electrode 106, and an Au film is deposited as thep electrode 107. In place of the optically transparent electrodematerial, Ni, Pd/Mo, Pd/Pt, Pd/Au or Ni/Au, for example, may also beemployed.

[0070] Finally, the chip is obtained by division using a scriber on theback surface (surface of the deposited n electrode 108) of the n-typeGaN substrate 100. Scribing is carried out from the back surface of thesubstrate so that no shavings resulting from scribing adheres to theoptically transparent electrode for extracting light. In relation to thedirection of scribing, the chip is so divided that at least a side ofeach device chip includes the cleavage plane of the nitridesemiconductor substrate. Therefore, the chip is prevented fromabnormality in shape resulting from chipping or cracking and thus theyield of device chips per wafer is improved.

[0071] In this Embodiment, the low-temperature buffer layer 101 may beof Al_(x)Ga_(1−x)N (0≦x≦1), and this buffer layer may be omitted.However, surface morphology of a currently available GaN substrate isnot satisfactory and hence it is more preferable to provide theAl_(x)Ga_(1−x)N buffer layer (0≦x≦1) so that the surface morphology isimproved. The term “low-temperature buffer layer” denotes a buffer layerformed at a relatively low growth temperature of 450° C. to 600° C. Thebuffer layer formed in such a low growth temperature range ispolycrystalline or amorphous.

[0072] While the single well layer 103 in this Embodiment is provided incontact between the n-type GaN layer 102 and the p-typeAl_(0.1)Ga_(0.9)N carrier blocking layer 104, a first new intermediatelayer may be provided between the n-type GaN layer 102 and the singlewell layer 103. Similarly, a second new intermediate layer may beprovided between the single well layer 103 and the p-typeAl_(0.1)Ga_(0.9)N carrier blocking layer 104. In this case, therefractive indices of these layers are set in the relation of the singlewell layer>the first intermediate layer>the n-type GaN layer and therelation of the single well layer>the second intermediate layer>thep-type AlGaN carrier blocking layer. Thus, the single well layer canefficiently confine light so that the light-emitting diode can beapplied to a super-luminescent diode or a graded-index separateconfinement heterostructure laser, for example. Although the impurity(Si) is added at concentration of 1×10¹⁸/cm³ to the single well layer103 in this Embodiment, it may be omitted.

[0073] In the p-type Al_(0.1)Ga_(0.9)N carrier blocking layer 104, thecomposition ratio of Al may be other than 0.1. When this Al compositionratio is increased, the carrier confinement effect in the single welllayer is preferably strengthened. When the Al composition ratio isreduced within a range holding the carrier confinement effect, on theother hand, carrier mobility in the carrier blocking layer is increasedto preferably reduce the electric resistivity. Further, the carrierblocking layer 104 containing Al can prevent the element of As, P or Sbcontained in the single well layer from diffusing into the p-type GaNcontact layer 105. Thus, it is possible to prevent the emissionwavelength of the light-emitting device from deviating from the designedvalue. The material for the carrier blocking layer 104 is not restrictedto the ternary mixed crystal of AlGaN but may alternatively be aquaternary mixed crystal of AlInGaN, AlGaNP or AlGaNAs.

[0074] Although the n electrode 108 in this Embodiment is formed on theback surface of the n-type GaN substrate 100, alternatively the n-typeGaN layer 102 may be partially exposed from the p electrode side of theepi-wafer by dry etching or the like to form the n electrode on theexposed part (see FIG. 4, for example).

[0075] While the C plane (0001) of the GaN substrate is utilized in thisEmbodiment, a C plane (000-1), an A plane {11-20}, an R plane {1-102},an M plane {1-100} or a {1-101} plane may alternatively be employed asthe main surface orientation of the substrate in place of the C plane.If the substrate surface has an off-angle of within 2 degrees from theplane orientation, the surface morphology is preferably improved.Further, the GaN substrate may be replaced with another nitridesemiconductor substrate.

[0076] While this Embodiment has been described with reference to acrystal growth method employing an MOCVD apparatus, molecular beamepitaxy (MBE) or hydride vapor phase epitaxy (HVPE) may alternatively beemployed.

Embodiment 2

[0077] Embodiment 2 is different from Embodiment 1 only in a point thatthe GaN substrate 100 of FIG. 1 is replaced with a pseudo GaN substrate200 of FIG. 2 or a pseudo GaN substrate 200 a of FIG. 3B while a pelectrode and an n electrode are formed on the same side of thesubstrate as shown in FIG. 4.

[0078] The pseudo GaN substrate 200 shown in FIG. 2 includes a seedsubstrate 201, a low-temperature buffer layer 202, an n-type GaN layer203, a growth inhibitor film 204 and an n-type GaN thick film 205.

[0079] The seed substrate 201 is used as a base for growing the n-typeGaN thick film 205. The term “growth inhibitor film” denotes a film onwhich no nitride semiconductor layer grows directly. The pseudo GaNsubstrate is not restricted to the structure shown in FIG. 2 but thisterm denotes a substrate including at least a seed substrate and agrowth inhibitor film.

[0080] The pseudo GaN substrate 200 a shown in FIG. 3B includes a seedsubstrate 201, a low-temperature buffer layer 202, a first n-type GaNfilm 203 a and a second n-type GaN film 203 b. FIG. 3A shows anintermediate step for preparing the pseudo GaN substrate 200 a.

[0081] When the pseudo GaN substrate 200 a is prepared, the first n-typeGaN film 203 a is stacked and thereafter the surface of the GaN film 203a is worked to form trenches by dry etching or wet etching. Thereafterthe wafer is introduced into a crystal growth apparatus again forstacking the second n-type GaN film 203 b and completing the pseudo GaNsubstrate 200 a (see FIG. 3B). Although the trenches is formed to onlyan intermediate depth of the first n-type GaN film 203 a in FIG. 3A, itmay alternatively be formed to a depth reaching the low-temperaturebuffer layer 202 or the seed substrate 201.

[0082] When a nitride semiconductor film is grown on the pseudo GaNsubstrate 200 or 200 a prepared in the aforementioned manner, thecrystal defect density of the nitride semiconductor film is low ascompared with that grown directly on a sapphire substrate or an SiCsubstrate. When the pseudo GaN substrate of this Embodiment is used,therefore, the effect of reducing crystal system separation can be moreefficiently attained by adding Al to a single well layer so that theluminous efficiency of the light-emitting device can be improved. Morespecifically, the material for the seed substrate 201 can be selectedfrom C-plane sapphire, M-plane sapphire, A-plane sapphire, R-planesapphire, GaAs, ZnO, spinel, Ge, Si, GaN, 6H—SiC, 4H—SiC and 3C—SiC.

[0083] When the seed substrate 201 is formed by an SiC substrate or anSi substrate, the n electrode may be formed on the back surface of thesubstrate as shown in FIG. 1 since this substrate is conductive. In thiscase, however, a high-temperature buffer layer must be formed in placeof the low-temperature buffer layer 202. The term “high-temperaturebuffer layer” denotes a buffer layer formed at a relatively high growthtemperature of at least 700° C. The high-temperature buffer layer mustcontain Al. This is because no nitride semiconductor film havingexcellent crystallinity can be formed on the SiC substrate or the Sisubstrate unless the high-temperature buffer layer contains at least Al.The most preferable material for the high-temperature buffer layer isINAlN.

[0084] The growth inhibitor film 204 can be formed specifically by adielectric film such as an SiO₂ film, an SiN, film, a TiO₂ film or anAl₂O₃ film or by a metal film such as a tungsten film.

Embodiment 3

[0085] Embodiment 3 is different from Embodiment 1 only in a point thata nitride semiconductor light-emitting diode is formed on a substrateother than a nitride semiconductor substrate through a nitridesemiconductor buffer layer while a p electrode and an n electrode areformed on the same side of the substrate.

[0086]FIG. 4 shows the nitride semiconductor light-emitting diodeaccording to Embodiment 3 as a schematic sectional view, and FIG. 5shows a top plan view corresponding to FIG. 4. The diode device shown inFIG. 4 includes a C-plane {0001} sapphire substrate 300, alow-temperature GaN buffer layer 101 (thickness: 25 nm), an n-type GaNlayer 102, a single well layer 103, a p-type Al_(0.1)Ga_(0.9)N carrierblocking layer 104, a p-type GaN contact layer 105, an opticallytransparent electrode 106, a p electrode 107, an n electrode 108 and adielectric film 109.

[0087] The nitride semiconductor light-emitting diode grown on thesubstrate (sapphire substrate) other than a nitride semiconductorsubstrate has higher crystal defect density (etch pit density of atleast 4×10⁸/cm²) as compared with that grown on the nitridesemiconductor substrate of Embodiment 1 or the pseudo GaN substrate ofEmbodiment 2. As compared with a conventional diode device including aGaNAs well layer, a GaNP well layer or a GaNSb well layer, however,crystal system separation is reduced and luminous intensity is improvedin the device according to Embodiment 3 which includes the well layercontaining Al.

[0088] While the sapphire substrate is employed in this Embodiment,6H—SiC, 4H:SiC, 3C—SiC, Si or spinel (Mg Al₂O₄) may alternatively beemployed as material for the substrate. The SiC substrate or the Sisubstrate is a conductive substrate, and hence the n electrode may beformed on the back surface of the substrate as shown in FIG. 1. When theSiC substrate or the Si substrate is employed, a high-temperature bufferlayer containing Al must be formed similarly to the case of Embodiment2.

[0089] While the C-plane {0001} substrate is employed in Embodiment 3,the plane orientation of the main surface of the substrate mayalternatively be an A plane {11-20}, an R plane {1-102} or an M plane{1-100}. Further, on a main surface of the substrate having an off-anglewithin 2 degrees from the plane orientation, the surface morphology isimproved.

Embodiment 4

[0090] In Embodiment 4, C (carbon) of 1×10²⁰/cm³ is added in place ofthe Si impurity in the single well layer in every aforementionedEmbodiment. Even when C is used in place of the impurity Si in the welllayer, a similar effect is attained.

Embodiment 5

[0091] In Embodiment 5, Mg of 1×10¹⁶/cm³ is added in place of the Siimpurity in the single well layer in every aforementioned Embodiment.Even when Mg is used in place of the impurity Si in the well layer, asimilar effect is attained.

Embodiment 6

[0092] In Embodiment 6, a nitride semiconductor light-emitting diodeincluding a single well layer according to the present invention isapplied to a light-emitting device (a display or a white light sourcedevice). The light-emitting diode according to the present invention canbe utilized for at least one of the three primary colors (red, green andblue) of light in the display.

[0093] For example, a conventional amber light-emitting diode includingan InGaN well layer has a high In composition ratio (remarkableinfluence of phase separation), and does not reach a commercializationlevel in view of reliability and luminous intensity. However, the singlewell layer according to the present invention has no influence of phaseseparation due to In and can reduce crystal system separation, and henceit is possible to prepare a light-emitting diode for a color of a longwavelength. A light-emitting diode according to the present inventionhaving another luminescent color can also be prepared with reference tothe aforementioned Embodiments and Tables 1 and 2.

[0094] The aforementioned light-emitting diodes of the three primarycolors according to the present invention can be utilized also in awhite light source device. When coated with a fluorescent paint, theinventive light-emitting diode having an emission wavelength within arange of 380 nm to 440 nm can be utilized as a white light sourcedevice. When the inventive light-emitting diode is utilized for a whitelight source in place of a halogen light source in a conventional liquidcrystal display, the white light source can be utilized as a backlighthaving low power consumption and high luminance. The white light sourcecan also be utilized as a backlight for a liquid crystal display of aman-machine interface in a portable notebook-type computer or a portabletelephone, and also enables provision of a miniature high-definitionliquid crystal display.

Embodiment 7

[0095] A nitride semiconductor laser device according to Embodiment 7 ofthe present invention is now described.

[0096] The nitride semiconductor laser device of Embodiment 7 shown in aschematic sectional view of FIG. 7 includes a C-plane (0001) sapphiresubstrate 700, a GaN buffer layer 701, an n-type GaN contact layer 702,an n-type In_(0.07)Ga_(0.93)N anti-cracking layer 703, an n-typeAl_(0.1)Ga_(0.9)N cladding layer 704, an n-type GaN light guide layer705, an emission layer 706, a p-type Al_(0.2)Ga_(0.8)N shielding layer707, a p-type GaN light guide layer 708, a p-type Al_(0.1)Ga_(0.9)cladding layer 709, a p-type GaN contact layer 710, an n-type electrode711, a p-type electrode 712 and an SiO₂ dielectric film 713.

[0097] In formation of the laser device shown in FIG. 7, the sapphiresubstrate 700 is set in an MOCVD apparatus to grow the GaN buffer layer701 to a thickness of 25 nm at a relatively low substrate temperature of550° C. with NH₃ (ammonia) as a raw material for N of the group Velement and TMGa (trimethyl gallium) as a raw material for Ga of thegroup III element. Then, SiH₄ (silane) is also utilized in addition toNH₃ and TMGa to grow the n-type GaN contact layer 702 (Si impurityconcentration: 1×10¹⁸/cm³) to a thickness of 3 μm at a temperature of1050° C. Then, the substrate temperature is reduced to about 700° C. to800° C. to grow the n-type In_(0.07)Ga_(0.93)N anti-cracking layer 703to a thickness of 40 nm with TMIn (trimethyl indium) as a raw materialfor In of the group III element. The substrate temperature is increasedto 1050° C. again to grow the n-type Al_(0.1)Ga_(0.9)N cladding layer704 (Si impurity concentration: 1×10¹⁸/cm³) of 0.8 μm thickness withTMAl (trimethyl aluminum) as a raw material for Al of the group IIIelement, and then grow the n-type GaN light guide layer 705 (Si impurityconcentration: 1×10¹⁸/cm³) to a thickness of 0.1 μm.

[0098] Thereafter the substrate temperature is reduced to 800° C. toform the emission layer 706 having a multiple quantum well structure byalternately stacking a plurality of GaN barrier layers each of 6 nmthickness and a plurality of Al_(0.03)Ga_(0.97)N_(0.97)P_(0.03) welllayers each of 4 nm thickness. According to this Embodiment, theemission layer 706 has a multiple quantum well structure starting with abarrier layer and ending with another barrier layer, and includes threequantum well layers. In growth of these barrier layers and well layers,SiH₄ is so added that both of these layers have Si impurityconcentration of 1×10¹⁸/cm³. A growth interruption period of at least 1second and not more than 180 seconds may be inserted between growth ofeach barrier layer and growth of each well layer or between growth ofeach well layer and growth of each barrier layer. In this case, thebarrier layers and the well layers are so improved in flatness that theemission half-band width can be reduced.

[0099] In order to obtain a target emission wavelength in the case ofemploying an AlGaNAs-based or AlGaNP-based semiconductor for the welllayers, any numerical value shown in the above Table 1 or 2 may beemployed as a value for a content x or y of As or P in relation to thecontent a of Al. When an AlGaNSb-based semiconductor is used for thewell layers, the Sb content in the group V elements is preferably notmore than about 4%, as described before. This is because the crystalsystem of the AlGaNSb semiconductor is readily separated into a cubicsystem having a high Sb content and a hexagonal system having a low Sbcontent if the AlGaNSb semiconductor contains Sb in concentration higherthan the upper limit.

[0100] After formation of the emission layer 706, the substrate isheated to 1050° C. again to successively grow the p-typeAl_(0.2)Ga_(0.8)N shielding layer 707 of 20 nm thickness, the p-type GaNlight guide layer 708 of 0.1 μm thickness, the p-type Al_(0.1)Ga_(0.9)Ncladding layer 709 of 0.5 μm thickness and the p-type GaN contact layer710 of 0.1 μm thickness. As to the p-type impurity, Mg can be addedthrough EtCP₃Mg (bisethylcyclopentadienyl magnesium) in concentration of5×10¹⁹ to 2×10²⁰/cm³.

[0101] The p-type impurity concentration in the p-type GaN contact layer710 is preferably increased as approaching the junction surface betweenthe same and the p-type electrode 712. Thereby, contact resistancebetween the p-type GaN contact layer 710 and the p-type electrode can befurther reduced. In order to remove residual hydrogen hinderingactivation of Mg as the p-type impurity in the p-type layers, a slightquantity of oxygen may be mixed during growth of the p-type layers.

[0102] After growth of the p-type GaN contact layer 710, all gas in thereaction chamber of the MOCVD apparatus is replaced with nitrogencarrier gas and NH₃, and the temperature is reduced at a cooling rate of60° C./min. Supply of NH₃ is stopped when the substrate temperature isreduced to 800° C., and this substrate temperature of 800° C. ismaintained for 5 minutes before subsequent cooling of the substrate tothe room temperature. Such a temporary holding temperature for thesubstrate is preferably in a range of 650° C. to 900° C., and theholding time is preferably in a range of 3 minutes to 10 minutes.Further, the cooling rate from the holding temperature to the roomtemperature is preferably at least 30° C./min.

[0103] As a matter of fact, according to Raman measurement, the surfaceof the film grown in the aforementioned manner already exhibited ap-type property, even though the same was not subjected to theconventional annealing for activating the p-type impurity. Further, whenthe p-type electrode 712 was formed as described later, contactresistance thereof was also reduced.

[0104] A process of working the epitaxial wafer taken out from the MOCVDapparatus into the laser device is now described.

[0105] First, part of the n-type GaN contact layer 702 is exposed by areactive ion etching apparatus, to form the n-type electrode 711consisting of layers stacked in order of Hf/Au. Alternatively, stackedlayers of Ti/Al, Ti/Mo, Hf/Al or the like can be employed as thematerial for the n-type electrode 711. Hf is effective for reducing thecontact resistance of the n-type electrode. Regarding the p-typeelectrode part, etching is carried out in a striped manner along the<1-100> direction of the sapphire substrate 700, the SiO₂ dielectricfilm 713 is deposited, the p-type GaN contact layer 710 is exposed andthe stacked layers of the order of Pd/Au is deposited thereby formingthe ridge-striped p-type electrode 712 of 2 μm width. Stacked layers ofNi/Au or Pd/Mo/Au can alternatively be employed as the material for thep-type electrode.

[0106] Finally, a Fabry-Perot resonator having a cavity length of 500 μmis prepared through cleavage or dry etching. This cavity length ispreferably in a range of 300 μm to 1000 μm in general. The end mirrorsurfaces of the resonator are formed to coincide with the M plane of thesapphire substrate (see FIG. 8). Cleavage and chip division of the laserdevice are performed from the substrate side with a scriber along brokenlines 2A and 2B shown in FIG. 8. Thus, flatness of the laser endsurfaces can be obtained while no shavings resulting from scribingadhere to the surface of the epitaxial layer, whereby the yield of thelight-emitting devices is improved.

[0107] The feedback method of the laser resonator is not restricted tothe Fabry-Perot method but generally known DFB (distributed feedback) orDBR (distributed Bragg reflection) can alternatively be employed, as amatter of course.

[0108] After formation of the mirror end surfaces of the Fabry-Perotresonator, dielectric films of SiO₂ and TiO₂ are alternately depositedon one of the mirror end surfaces for forming a reflective dielectricmulti-layered film having a reflectance of 70%. Alternatively, amulti-layered film of SiO₂/Al₂O₃ or the like can also be employed as thereflective dielectric multi-layered film.

[0109] The reason why the part of the n-type GaN contact layer 702 isexposed by reactive ion etching is that the insulating sapphiresubstrate 700 is used. In the case of using a conductive substrate suchas a GaN substrate or an SiC substrate, therefore, no part of the n-typeGaN layer 702 may be exposed but the n-type electrode may be formed onthe back surface of the conductive substrate.

[0110] A method of mounting the aforementioned laser chip on a packageis now described. When the laser including the aforementioned emissionlayer is employed in view of the characteristics thereof as abluish-purple (wavelength: 410 nm) high-output (50 mW) laser suitablefor an optical disk for high-density recording, attention must be givento heat dissipation since the sapphire substrate has low thermalconductivity. For example, it is preferable to connect the chip to apackage body with an In solder material while the semiconductor junctionis directed downward. The chip may not be directly mounted on thepackage body or a heat sink part but may be bonded thereto through asubmount of Si, AlN, diamond, Mo, CuW, BN, Cu, Au or Fe having goodthermal conductivity.

[0111] When the nitride semiconductor laser including the aforementionedemission layer is prepared on an SiC substrate, a nitride semiconductorsubstrate (e.g., a GaN substrate) or a GaN thick-film substrate (e.g.,obtained by grinding the seed substrate 801 away from the substrate 800of FIG. 14) having high thermal conductivity, the chip can also beconnected to the package body with the In solder material while thesemiconductor junction is directed upward, for example. Also in thiscase, the substrate of the chip may not be directly mounted on thepackage body or the heat sink part but may be connected through asubmount of Si, AlN, diamond, Mo, CuW, BN, Cu, Au or Fe.

[0112] As described above, it is possible to prepare the laser utilizingthe nitride semiconductor containing Al for the well layers included inthe emission layer.

[0113] More detailed description is now made in relation to the emissionlayer 706 included in the laser according to the aforementionedEmbodiment.

[0114] In the case of preparing a light-emitting device utilizing aconventional InGaN quantum well layer, the chemical thermal equilibriumstate of the InGaN layer is so unstable that it is difficult to form anemission layer having excellent crystallinity, as described before.Particularly in the case of growing an InGaN crystal layer having an Incontent of at least 15% in the group III elements, the InGaN crystal isreadily concentration-separated into regions having high and low Incontents depending on the growth temperature. Such concentrationseparation causes reduction of luminous efficiency and increase of thehalf width of the emission wavelength (color heterogeneity). On theother hand, the GaNAs well layer disclosed in Japanese PatentLaying-Open 10-270804 contains no In and thus causes no problem of theaforementioned concentration separation, but it contains As which isliable to cause crystal system separation into a hexagonal system and acubic system and then reduce the crystallinity and the luminousefficiency.

[0115] The AlGaNAsPSb well layer according to the present invention,containing Al in place of In while containing at least any element ofAs, P and Sb, can realize a target emission wavelength by adjusting thecontents (see Tables 1 and 2). Therefore, the well layer according tothe present invention is exraneous to the aforementioned concentrationseparation related to In. For example, a conventional amberlight-emitting diode has a high In content (i.e., remarkable influenceof phase separation) in its InGaN well layer, and thus does not reach acommercialization level in view of its reliability and luminousintensity. However, the AlGaNAsPSb well layer according to the presentinvention containing no In causes no problem of concentration separationin relation to In, but can enable preparation of a light-emitting devicecapable of emitting long wavelength light shown in Table 1 or 2.

[0116] The AlGaNAsPSb well layer according to the present inventioncontains Al, differently from the conventional GaNAs well layer (As isat least partially replaceable with P and/or Sb: this also applies tothe following description). In other words, the crystal systemseparation being problematic in the conventional GaNAs well layer can besuppressed by introducing Al as in the present invention. This crystalsystem separation conceivably results from the fact that adhesion of As(this also applies to P or Sb) with respect to the group III elements isvery high as compared with N and that N has extremely high volatility(i.e., N readily escapes from the crystal) as compared with As (thisalso applies to P or Sb). Therefore, it is conceivably possible tosuppress crystal system separation by adding Al of the group III elementhaving very high reactivity thereby capturing N and preventing N fromescaping from the grown crystal. Further, the well layer according tothe present invention, containing at least any element of As, P and Sb,can reduce the effective mass of electrons and holes and improve carriermobility.

[0117] Thus, when the well layer according to the present invention isutilized for a light-emitting device, it is possible to implement along-lived light-emitting device having low power consumption and highoutput due to the high crystallinity in the well layer and the reductionof the effective mass of carriers.

[0118] The Al content in the inventive well layer is now described.First, the inventors have investigated what degree of As, P or Sbcontent causes the aforementioned crystal system separation. As aresult, the crystal system separation started to take place when As, Por Sb was added into a GaN crystal in concentration of 1×10¹⁸/cm³(crystal system separation ratio of about 2 to 3%), and the crystalsystem separation ratio reached about 13 to 15% when the content wasabout 10% of the group V elements in the well layer. The term “crystalsystem separation ratio” denotes the volume ratio of crystal systemseparation regions to normal regions having an average composition ratiowithout crystal system separation, in the unit volume of the well layer.

[0119] In relation to the well layer according to the present invention,influence exerted by the Al content on the crystal system separationratio and the luminous intensity has already been considered withreference to FIG. 6.

[0120] The emission layer according to the present invention preferablyhas a multiple quantum well structure obtained by alternately stacking aplurality of quantum well layers and a plurality of barrier layers. Thisis because threshold current density is reduced in a laser (see FIG. 9)and luminous intensity is improved in a light-emitting diode (see FIG.16) by employing the multiple quantum well structure. Such advantagesresulting from employment of the multiple quantum well structure can beremarkably and reliably attained due to the addition of Al according tothe present invention. This is because crystal system separation of thewell layers is suppressed and interfacial steepness between the welllayers and the barrier layers is improved when Al is added to the welllayers containing at least one of As, P and Sb. In a conventional GaNAswell layer containing no Al, for example, regions having differentcrystal systems are mixed therein and hence interfacial steepnessbetween the well layer and a barrier layer is remarkably deteriorated asthe number of stacked layers is increased. Such deterioration of theinterfacial steepness makes it difficult to form the multiple quantumwell structure, and causes color heterogeneity and reduction of luminousintensity in the light-emitting device. The present invention enablesformation of the multiple quantum well structure without reducinginterfacial steepness by adding Al into the well layers.

[0121] The relation between the well layer and the barrier layerconstituting the emission layer is now described. AnAl_(a)Ga_(1−a)N_(1−x-y-z)As_(x)P_(y)Sb_(x) (023 x≦0.10, 0≦y≦0.16,0≦z≦0.04, x+y+z>0) well layer according to the present invention,causing neither concentration separation nor crystal system separationdescribed above, can be grown to a thickness of about 300 nm dependingon the Al content if the contents of As, P and Sb are within therespective limited ranges. For a light-emitting device utilizing themultiple quantum well effect, however, a thickness of the well layer ispreferably in a range of 0.4 to 20 nm. The reason why the lower limit is0.4 nm is that no light emission is attained unless the thickness of thewell layer is in excess of this level.

[0122] The most preferable barrier layer for theAl_(a)Ga_(1−a)N_(1−x-y-z)As_(x)P_(y)Sb_(z) well layer is a nitridesemiconductor barrier layer containing none of As, P and Sb. If thebarrier layer itself contains none of As, P and Sb, it causes no crystalsystem separation. This means that the barrier layer causes no hindrancein formation of the multiple quantum well structure.

[0123] A barrier layer of InGaN, GaN, InAlGaN or AlGaN can be employedas the nitride semiconductor barrier layer containing none of As, P andSb. The growth temperature for an InGaN barrier layer containing In canbe reduced to about that for the well layer, and the crystallinitythereof is improved. In order to suppress concentration separationrelated to In, however, the In content must be set to less than 15% ofthe group III elements. A GaN barrier layer containing no In causes noconcentration separation. However, the crystallinity is deteriorated ifthe growth temperature therefor is low, and hence it is important to setthe growth temperature as high as possible. An InAlGaN barrier layercontaining Al can stably grow also at a high growth temperature. Thisbarrier layer contains In and hence the growth temperature therefor canbe reduced to about that for the well layer. Also in this case, however,the In content must be set to less than 15% of the group III elements.An AlGaN barrier layer is deteriorated in crystallinity unless the sameis grown at a high temperature, and hence it is desirable to reduce theAl content to the minimum (not more than 10% of the group II elements)within its allowable range and to increase the growth temperaturetherefor to the maximum within its allowable range.

[0124] A nitride semiconductor barrier layer containing at least any ofAs, P and Sb is now described. In spite of the above description, anadvantage of taking a risk of introducing As, P and/or Sb into thebarrier layer resides in that the refractive index of the barrier layercontaining As, P and/or Sb tends to increase and hence light confinementefficiency is so improved as to reduce lasing threshold current densityor improve optical properties. A barrier layer of InAlGaNAs, InAlGaNP,InAlGaNSb, InAlGaNAsP, InAlGaNAsPSb, AlGaNAs, AlGaNP, AlGaNSb, AlGaNAsP,AlGaNAsPSb, GaNAs, GaNP, GaNSb, GaNAsP, GaNAsPSb, InGaNAs, InGaNP,InGaNSb, InGaNAsP or InGaNAsPSb, for example, can be employed as thenitride semiconductor barrier layer containing at least any of As, P andSb.

[0125] Among these barrier layers, that containing Al can suppressinfluence of crystal system separation similarly to the inventive welllayer. In each barrier layer containing In, however, it is necessary toset the In content to less than 15% of the group III elements in orderto suppress concentration separation of In. In each barrier layercontaining no Al, the content of As, P and/or Sb in the group V elementsmust be suppressed low in order to suppress crystal system separation.According to investigation by the inventors, however, since the barrierlayer does not directly emit light due to recombination of injectedcarriers differently from the well layer, it exhibited larger tolerancewith respect to the crystal system separation ratio as compared with thewell layer. The tolerance of As is not more than about 5%, that of P isnot more than about 6% and that of Sb is not more than about 3% in thegroup V elements. Each barrier layer containing In can preferably reduceits energy band gap thereby suppressing the contents of As, P and Sb(i.e., the crystal separation ratio can be reduced). Also in this case,however, the In content must be set to less than 15% of the group IIIelements in order to suppress In concentration separation.

[0126] A thickness of the barrier layer is preferably in a range of 1 to20 nm. The number of the barrier layers in the multiple quantum wellstructure is naturally adjusted in relation to the number of the welllayers, since the well layers and the barrier layers are alternatelystacked.

[0127] In relation to addition of the impurity to the emission layer,while SiH₄ (Si) is added to both of the well layers and the barrierlayers as the impurity in this Embodiment, the impurity mayalternatively be added to only either layers, or lasing is possible alsowhen no impurity is added to the layers. As a result ofphotoluminescence (PL) measurement, however, PL luminous intensity wasstrengthened in a range of about 1.2 times to about 1.4 times in thecase of adding SiH₄ to both of the well layers and the barrier layers ascompared with the case of adding no impurity. Thus, it is preferable toadd an impurity such as SiH₄ (Si) into the emission layer in thelight-emitting diode. Since the inventive well layer is formed of anAlGaNAsPSb mixed crystal system containing absolutely no In, it forms nolocalized energy levels due to In differently from the conventionalInGaN mixed crystal and then the luminous intensity conceivably dependsstrongly on the crystallinity of the well layer. Therefore, it isdesired to improve the crystallinity of the emission layer by adding animpurity such as Si. In other words, such an impurity forms nuclei forcrystal growth and the well layer is crystal-grown from the nucleithereby improving the crystallinity. Although Si (SiH₄) is added inconcentration of 1×10¹⁸/cm³ according to this Embodiment, a similareffect is attained also when adding O, S, C, Ge, Zn, Cd or Mg in placeof Si. The concentration of the added atoms is preferably in a range ofabout 1×10¹⁶ to 1×10²⁰/cm³.

[0128] Generally in the case of a laser, when modulation doping iscarried out by adding an impurity only to barrier layers, the thresholdcurrent density is reduced due to less carrier absorption in welllayers. Nevertheless, when the impurity is added into the inventive welllayers, the threshold of the laser is rather reduced. This isconceivably because this Embodiment progresses crystal growth startingfrom the sapphire substrate different from the nitride semiconductorsubstrate and hence the number of crystal defects is so large (threadingdislocation density: about 1×10¹⁰/cm²) that it is more effective forreducing the laser threshold current density to improve thecrystallinity by adding the impurity than considering carrier absorptiondue to the impurity in the well layers.

[0129]FIG. 9 shows relation between the number of the well layersincluded in the emission layer (multiple quantum well structure) and thelaser threshold current density. The horizontal axis of this graph showsthe numbers of the well layers, and the vertical axis shows thethreshold current density (arbitrary unit). Marks ∘ show laser thresholdcurrent density values in the case of employing a sapphire substrate,and marks  show those in the case of employing a GaN substrate. Whenthe number of the well layers is not more than 10, continuous lasing atthe room temperature is enabled. In order to further reduce the lasingthreshold current density, the number of the well layers is preferablyat least 2 and not more than 5. Further, it is understood that thethreshold current density is reduced by using not the sapphire substratebut the GaN substrate.

[0130] The p-type AlGaN shielding layer 707 and the p-type layer 708 arestacked in this order on the emission layer 706. This p-type layer 708corresponds to a p-type light guide layer in the case of a laser, andcorresponds to a p-type cladding layer or a p-type contact layer in thecase of a light-emitting diode.

[0131] As a result of PL measurement, the shift quantity from a designedemission wavelength was smaller and PL luminous intensity was strongerin the case having the shielding layer 707 as compared with in the caseof having no shielding layer. As compared with the emission layer 706,the growth temperature for the p-type layer 708 provided thereon is sohigh that escape of N takes place particularly in barrier layerscontaining no Al and then it consequently acts to promote the crystalsystem separation. By providing the shielding layer 707 containing Al atthe interface between the emission layer and the p-type layer providedthereon, however, it is possible to suppress N escape and crystal systemseparation and then to prevent propagation of influence (such as crystalsystem separation) from the emission layer 706 to the p-type layer 708.Particularly when the emission layer 706 having the multiple quantumwell structure has an energy band structure of FIG. 10A starting with abarrier layer and ending with another barrier layer, the effect of theshielding layer 707 is remarkably recognized.

[0132] As seen from the above, it is important that the shielding layer707 contains at least Al. Further, the polarity of the shielding layeris preferably the p type. This is because the position of the p-njunction in the vicinity of the emission layer is changed thereby toreduce the luminous efficiency unless the shielding layer is of the ptype.

[0133] Similarly to the above case, an n-type AlGaN shielding layer maybe provided to be in contact between the emission layer 706 and then-type layer 705. This n-type layer 705 corresponds to an n-type lightguide layer in the case of a laser, and corresponds to an n-typecladding layer or an n-type contact layer in the case of alight-emitting diode. The effect of such an n-type AlGaN shielding layeris substantially similar to that of the p-type AlGaN shielding layer707.

[0134] As the band gap structure of the emission layer, it is possibleto employ the structure exemplarily illustrated in FIG. 12 or FIG. 10A.FIG. 12 illustrates a case where light guide layers and barrier layersare made of the same nitride semiconductor material. As illustrated inFIG. 10A, however, the band gaps of the light guide layers and thebarrier layers may be different from each other.

[0135] More specifically, the energy band gap of the barrier layers isrendered smaller as compared with that of the light guide layer, asshown in FIG. 10A. Then, a multiple quantum well effect resulting fromsub-bands is readily attained as compared with the case shown in FIG.12, while the refractive index of the barrier layers exceeds that of thelight guide layer to improve the light confinement effect, so that thecharacteristic (unimodality) of the vertical transverse mode can beimproved. Particularly when the barrier layers contain As, P or Sb, therefractive index thereof preferably tends to increase remarkably.

[0136] Two types of structures are possible for the aforementionedemission layer rendering the energy band gap of the barrier layerssmaller as compared with the light guide layer, as shown in FIGS. 10Aand 10B. In other words, the emission layer having the multiple quantumwell structure may have either the structure starting with a barrierlayer and ending with another barrier layer or the structure startingwith a well layer and ending with another well layer. FIGS. 11A and 11Bshow band gap structures of the emission layer with no shielding layer.

Embodiment 8

[0137] In Embodiment 8, nitride semiconductor materials for well layersand barrier layers are varied in the emission layer having the multiplequantum well structure described with reference to Embodiment 7. Table 3shows the combinations of the nitride semiconductor materials for thewell layers and the barrier layers. TABLE 3 Well Layer AlGaNAs AlGaNPAlGaNSb Barrier GaN ◯ ◯ ◯ Layer GaNAs ◯ ◯ ◯ GaNP ◯ ◯ ◯ GaNSb ◯ ◯ ◯ InGaN◯ ◯ ◯ InGaNAs ◯ ◯ ◯ InGaNP ◯ ◯ ◯ InGaNSb ◯ ◯ ◯ AlGaN ◯ ◯ ◯ AlGaNAs ◯ ◯ ◯AlGaNP ◯ ◯ ◯ AlGaNSb ◯ ◯ ◯ InAlGaN ◯ ◯ ◯ InAlGaNAs ◯ ◯ ◯ InAlGaNP ◯ ◯ ◯InAlGaNSb ◯ ◯ ◯

[0138] Referring to Table 3, marks ∘ show preferable combinations ofnitride semiconductor materials for the well layers and the barrierlayers. The well layer contains any element As, P or Sb in Table 3, butit may alternatively contain a plurality of such elements. In otherwords, the well layer may be made of mixed crystals ofAlGaN_(1−x-y-z)As_(x)P_(y)Sb_(z) (0≦x≦0.10, 0≦y≦0.16, 0≦z≦0.04,x+y+z>0). The other remaining conditions related to the emission layerutilizing these nitride semiconductor materials are similar to those inthe case of Embodiment 7.

Embodiment 9

[0139] In Embodiment 9 shown in FIG. 13, an n-type GaN substrate 700 ahaving the C plane (0001) as its main surface is employed in place ofthe sapphire substrate 700 used in Embodiment 7. In the case of usingthe GaN substrate 700 a, an n-type GaN layer 702 may be directly grownon the GaN substrate with a GaN buffer layer 701 being omitted. However,crystallinity and surface morphology of a GaN substrate commerciallyavailable at present are not sufficiently satisfactory and hence it israther preferable to insert the GaN buffer layer 701 in order to improvethese.

[0140] In this Embodiment 9 using the n-type GaN substrate 700 a, ann-type electrode 711 can be formed on the back surface of the GaNsubstrate 700 a. Further, cleaved end surfaces of the GaN substrate areso smooth that a Fabry-Perot resonator having a cavity length of 300 μmcan be prepared with low mirror loss. Similarly to the case ofEmbodiment 7, the cavity length is preferably in a range of 300 μm to1000 μm in general. The mirror end surfaces of the resonator are formedcorresponding to a {1-100} plane of the GaN substrate 700 a. Cleavageand chip division into the laser device are performed from the substrateside with a scriber similarly to the aforementioned case of FIG. 8. Itis also possible to employ the aforementioned DFB or TBR as thefeedback- method for the laser resonator as a matter of course, and itis needless to say that a reflective dielectric multi-layered film maybe formed on one of the mirror end surfaces similarly to the case ofEmbodiment 7.

[0141] When the GaN substrate is employed in place of the sapphiresubstrate, thicknesses of an n-type AlGaN cladding layer 704 and ap-type AlGaN cladding layer 709 can be increased without causing cracksin the epitaxial wafer. Thicknesses of these AlGaN cladding layers arepreferably set in a range of 0.7 to 1.5 μm. Thereby, the verticaltransverse mode is rendered unimodal and the light confinement effect isimproved and then it become possible to improve optical characteristicsof the laser device and reduce the laser threshold current density.

[0142] The characteristics of the well layers included in the emissionlayer according to the present invention strongly depend on thecrystallinity (crystal defects) of the well layers as described before.When the nitride semiconductor laser device including the well layers isprepared with the GaN substrate as in this Embodiment, therefore,crystal defect density (e.g., threading dislocation density) in theemission layer is reduced and the lasing threshold current density isreduced in a range of 10% to 20% as compared with Embodiment 7 using thesapphire substrate.

[0143] The other remaining conditions related to the emission layer inthis Embodiment are similar to those in the case of Embodiment 7. Inrelation to impurity concentration in the emission layer, however, laserthreshold current density is reduced as compared with Embodiment 7 byemploying modulation doping of adding an impurity only into barrierlayers or by adding an impurity in concentration of 3×10¹⁸/cm³ into thewell layers. This is conceivably because the crystallinity of theemission layer is improved as compared with the case of employing thesapphire substrate, as described above.

Embodiment 10

[0144] Embodiment 10 is similar to Embodiment 7 or Embodiment 9 exceptthat the sapphire substrate 700 of Embodiment 7 was replaced with asubstrate 800 shown in FIG. 14. The substrate 800 of FIG. 14 includes aseed substrate 801, a buffer layer 802, an n-type GaN film 803, adielectric film 804 and an n-type GaN thick film 805 successivelystacked in this order.

[0145] In preparation of such a substrate 800, the buffer layer 802 isfirst stacked on the seed substrate 801 by MOCVD at a relatively lowtemperature of 550° C. The n-type GaN film 803 of 1 μm thickness isformed thereon being doped with Si at a temperature of 1050° C.

[0146] The wafer formed with the n-type GaN film 803 is taken out fromthe MOCVD apparatus to form the dielectric film 804 in a thickness of100 nm by sputtering, CVD or EB deposition, and the dielectric film 804is worked into a periodic striped pattern. These stripes formed along a<1-100> direction of the n-type GaN film 803 have a periodic pitch of100 μm and a stripe width of 5 μm in a <11-20> direction perpendicularto the <1-100> direction.

[0147] Then, the wafer with the dielectric film 804 worked in a stripedmanner is set in an HVPE apparatus so that the n-type GaN thick film 805having Si concentration of 1×10¹⁸/cm³ and a thickness of 350 μm isdeposited at a growth temperature of 1100° C.

[0148] The wafer formed with the n-type GaN thick film 805 is taken outfrom the HVPE apparatus so that a laser similar to that of Embodiment 7(see FIG. 7) is prepared thereon. In this Embodiment 10, however, thelaser is so prepared that a ridge strip part 1A thereof is not locatedimmediately above lines 810 and 811 in FIG. 8. This is in order toprepare the laser device on a part having small threading dislocationdensity (i.e., crystal defect density). The characteristics of the laserof Embodiment 10 prepared in this manner are basically similar to thosein the case of Embodiment 9.

[0149] The substrate 800 may be employed as a substrate for a laserafter removing the seed substrate 801 by a grinder. Further, thesubstrate 800 may be employed as a substrate for a laser after removingthe buffer layer 802 and the layer thereunder by a grinder. In addition,the substrate 800 may be employed as a substrate for a laser afterremoving the dielectric film 804 and all the layers thereunder by agrinder. When the seed substrate 801 is removed, an n-type electrode 711can be formed on the back surface of the substrate similarly to the caseof Embodiment 9. It is also possible to remove the seed substrate 801after preparing the laser.

[0150] In preparation of the aforementioned substrate 800, the seedsubstrate 801 may be made of any material such as C-plane sapphire,M-plane sapphire, A-plane sapphire, R-plane sapphire, GaAs, ZnO, MgO,spinel, Ge, Si, 6H—SiC, 4H—SiC or 3C—SiC. Any of a GaN layer, an AlNlayer, an Al_(x)Ga_(1−x)N (0<x<1) layer or an In_(y)Ga_(1−y)N (0<y≦1)layer grown at a relatively low temperature of 450° C. to 600° C. may beemployed as the buffer layer 802. An n-type Al_(z)Ga_(1−z)N (0<z<1) filmis employable in place of the n-type GaN film 803. Any of an SiO₂ film,an SiN_(x) film, a TiO₂ film and an Al₂O₃ film may be employed as thedielectric film 804. The n-type GaN thick film 805 may be replaced withan n-type Al_(w)Ga_(1−w)N (0<w≦1) thick film, and a thickness thereofmay be at least 20 μm.

Embodiment 11

[0151] Embodiment 11 relates to a nitride semiconductor light-emittingdiode device. FIG. 15A is a schematic vertical sectional view of anitride semiconductor light-emitting diode device of Embodiment 11, andFIG. 15 is a top plan view corresponding to FIG. 15A.

[0152] The light-emitting diode device of FIG. 15A includes a C-plane(0001) sapphire substrate 900, a GaN buffer layer 901 (thickness: 30nm), an n-type GaN contact layer 902 (thickness: 3 μm, Si impurityconcentration: 1×10¹⁸/cm³), an n-type Al_(0.1)Ga_(0.9)N shielding andcladding layer 903 (thickness: 20 nm, Si impurity concentration:1×10¹⁸/cm³), an emission layer 904, a p-type Al_(0.2)Ga_(0.8)N shieldingand cladding layer 905 (thickness: 20 nm, Si impurity concentration:6×10¹⁹/cm³), a p-type GaN contact layer 906 (thickness: 200 nm, Mgimpurity concentration: 1×10²⁰/cm³), an optically transparent p-typeelectrode 907, a pad electrode 908, an n-type electrode 909 and adielectric film 910.

[0153] In such a light-emitting diode device, however, the n-typeAl_(0.1)Ga_(0.9)N shielding and cladding layer 903 may alternatively beomitted. The p-type electrode 907 is made of Ni or Pd, the pad electrode908 is made of Au, and the n-type electrode 909 can be formed by alaminate of Hf/Au, Ti/Al, Ti/Mo or Hf/Al.

[0154] In the emission layer of this Embodiment, SiH₄ (Si impurityconcentration: 5×10¹⁷/cm³) is added to all the well layers and barrierlayers. Nitride semiconductor materials for these well layers andbarrier layers are similar to those in the case of Embodiment 7. Aneffect similar to that of Embodiment 9 is attained when a GaN substrateis employed in place of the sapphire substrate 900, and an effectsimilar to that of Embodiment 10 is also attained when the substrateshown in FIG. 14 is employed. Further, since the GaN substrate is aconductive substrate, both of the p-type electrode 907 and the n-typeelectrode 909 may be formed on one side of the light-emitting device asshown in FIG. 15B, or the n-type electrode may be formed on the backsurface of the GaN substrate while the optically transparent p-typeelectrode is formed on the epitaxial outermost surface.

[0155] Conditions related to the well layers and the barrier layersincluded in the emission layer 904 in this Embodiment 11 are similar tothose in the case of Embodiment 7.

[0156]FIG. 16 shows relation between the number of the well layersincluded in the emission layer of the light-emitting diode device andthe luminous intensity. The horizontal axis of this graph shows thenumbers of well layers, and the vertical axis shows the luminousintensity (normalized arbitrary unit). In FIG. 16, the luminousintensity is mormalized with reference (broken line) to the case ofemploying conventional InGaN well layers in place of GaNP well layers(may be well layers of GaNAs or GaNSb). Marks ∘ in the graph showluminous intensity values in the case of employing a sapphire substrate,and marks  show luminous intensity values in the case of employing aGaN substrate. It is understood from this graph that the preferrednumber of the well layers included in the light-emitting diode is atleast 2 and not more than 10. It is also understood that the luminousintensity is rather improved by employing the GaN substrate than thesapphire substrate.

Embodiment 12

[0157] Embodiment 12 relates to a nitride semiconductorsuper-luminescent diode device. Regarding this light-emitting device,its structure and a crystal growth method are similar to those in thecase of Embodiment 7 (see FIG. 7). Nitride semiconductor materials forwell layers and barrier layers included in an emission layer are similarto those in Embodiment 8. Also in this Embodiment, an effect similar tothat of Embodiment 9 is attained by employing a GaN substrate in placeof a sapphire substrate, and an effect similar to that of Embodiment 10is attained by employing the substrate shown in FIG. 14. Relationbetween the number of the well layers included in the emission layer andthe luminous intensity is similar to that in the case of Embodiment 11.

Embodiment 13

[0158] In Embodiment 13, C of 1×10²⁰/cm³ is added to the well layers andthe barrier layers in the emission layer of each of Embodiments 7 and 9to 11 in place of the impurity Si. Then, a similar effect is attainedalso by employing C in place of the impurity Si in the well layers andthe barrier layers.

Embodiment 14

[0159] In Embodiment 14, Mg of 1×10¹⁶/cm³ is added to the well layersand the barrier layers in the emission layer of each of Embodiments 7and 9 to 11 as the impurity in place of Si. Then, a similar effect isattained also by employing Mg as the impurity in place of Si in the welllayers and the barrier layers.

Embodiment 15

[0160] In Embodiment 15, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.1)Ga_(0.9)N_(0.97)As_(0.01) well layers (each thickness: 4nm)/In_(0.05)Ga_(0.95)N barrier layers (each thickness: 8 nm) of threecycles. Then, an effect similar to that of each of Embodiments 7 and 9to 11 is attained in Embodiment 15 also.

Embodiment 16

[0161] In Embodiment 16, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.05)Ga_(0.95)As_(0.01) well layers (thickness: 2 nm)/GaN barrierlayers (thickness: 4 nm) of five cycles. Then, an effect similar to thatof each of Embodiments 7 and 9 to 11 is attained in Embodiment 16 also.

Embodiment 17

[0162] In Embodiment 17, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.2)Ga_(0.8)N_(0.96)P_(0.04) well layers (thickness: 4 nm)/GaNbarrier layers (thickness: 7 nm) of three cycles. Then, an effectsimilar to that of each of Embodiments 7 and 9 to 11 is attained inEmbodiment 17 also.

Embodiment 18

[0163] In Embodiment 18, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.2)Ga_(0.8)N_(0.97)As_(0.03) well layers (thickness: 4nm)/Al_(0.1)Ga_(0.9)N_(0.99)P_(0.01) barrier layers (thickness: 10 nm)of four cycles. Then, an effect similar to that of each of Embodiments 7and 9 to 11 is attained in Embodiment 18 also.

Embodiment 19

[0164] In Embodiment 19, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.01)Ga_(0.99)N_(0.98)P_(0.02) well layers (thickness: 4nm)/Al_(0.01)In_(0.06)Ga_(0.93)N barrier layers (thickness: 8 nm) ofthree cycles. Then, an effect similar to that of each of Embodiments 7and 9 to 11 is attained in Embodiment 19 also.

Embodiment 20

[0165] In Embodiment 20, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.01)Ga_(0.99)N_(0.99)As_(0.01) well layers (thickness: 4 nm)/GaNbarrier layers (thickness: 3 nm) of six cycles. Then, an effect similarto that of each of Embodiments 7 and 9 to 11 is attained in Embodiment20 also.

Embodiment 21

[0166] In Embodiment 21, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.03)Ga_(0.97)N_(0.97)P_(0.03) well layers (thickness: 6nm)/In_(0.1)Al_(0.01)Ga_(0.89)N barrier layers (thickness: 3 nm) of fourcycles. Then, an effect similar to that of each of Embodiments 7 and 9to 11 is attained in Embodiment 21 also.

Embodiment 22

[0167] In Embodiment 22, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.03)Ga_(0.97)N_(0.98)As_(0.02) well layers (thickness: 4nm)/In_(0.01)Ga_(0.99)N_(0.99)As_(0.01) barrier layers (thickness: 10nm) of five cycles. Then, an effect similar to that of each ofEmbodiments 7 and 9 to 11 is attained in Embodiment 22 also.

Embodiment 23

[0168] In Embodiment 23, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.1)Ga_(0.9)N_(0.97)As_(0.03) well layers (thickness: 4 nm)/GaNbarrier layers (thickness: 4 nm) of six cycles. Then, an effect similarto that of each of Embodiments 7 and 9 to 11 is attained in Embodiment23 also.

Embodiment 24

[0169] In Embodiment 24, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.01)Ga_(0.99)N_(0.98)Sb_(0.02) well layers (thickness: 5 nm)/GaNbarrier layers (thickness: 5 nm) of three cycles. Then, an effectsimilar to that of each of Embodiments 7 and 9 to 11 is attained inEmbodiment 24 also.

Embodiment 25

[0170] In Embodiment 25, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.05)Ga_(0.95)N_(0.93)P_(0.07) well layers (thickness: 4nm)/In_(0.02)Al_(0.03)Ga_(0.95)N_(0.97)As_(0.03) barrier layers(thickness: 8 nm) of four cycles. Then, an effect similar to that ofeach of Embodiments 7 and 9 to 11 is attained in Embodiment 25 also.

Embodiment 26

[0171] In Embodiment 26, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.05)Ga_(0.95)N_(0.96)As_(0.04) well layers (thickness: 15nm)/GaN_(0.98)As_(0.02) barrier layers (thickness: 10 nm) of threecycles. Then, an effect similar to that of each of Embodiments 7 and 9to 11 is attained in Embodiment 26 also.

Embodiment 27

[0172] In Embodiment 27, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.01)Ga_(0.99)N_(0.92)P_(0.08) well layers (thickness: 5nm)/Al_(0.03)Ga_(0.97)N_(0.98)Sb_(0.02) barrier layers (thickness: 5 nm)of three cycles. Then, an effect similar to that of each of Embodiments7 and 9 to 11 is attained in Embodiment 27 also.

Embodiment 28

[0173] In Embodiment 28, the well layers and the barrier layers includedin the emission layer in each of Embodiments 7 and 9 to 11 are changedto Al_(0.01)Ga_(0.99)N_(0.95)As_(0.05) well layers (thickness: 6nm)/In_(0.15)Ga_(0.85)N_(0.98)P_(0.02) barrier layers (thickness: 6 nm)of two cycles. Then, an effect similar to that of each of Embodiments 7and 9 to 11 is attained in Embodiment 28 also.

Embodiment 29

[0174] In Embodiment 29, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.01)Ga_(0.99)N_(0.94)As_(0.06) well layers (thickness: 10nm)/In_(0.1)Al_(0.1)Ga_(0.8)N_(0.95)As_(0.05) barrier layers (thickness:4 nm) of four cycles. Then an effect similar to that of each ofEmbodiments 7 and 9 to 11 is attained in Embodiment 29 also.

Embodiment 30

[0175] In Embodiment 30, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.03)Ga_(0.97)N_(0.88)P_(0.12) well layers (thickness: 10nm)/Al_(0.1)Ga_(0.9)N_(0.93)As_(0.07) barrier layers (thickness: 15 nm)of four cycles. Then, an effect similar to that of each of Embodiments 7and 9 to 11 is attained in Embodiment 30 also.

Embodiment 31

[0176] In Embodiment 31, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.03)Ga_(0.97)N_(0.93)As_(0.07) well layers (thickness: 20nm)/GaN_(0.9)P_(0.1) barrier layers (thickness: 20 nm) of three cycles.Then, an effect similar to that of each of Embodiments 7 and 9 to 11 isattained in Embodiment 31 also.

Embodiment 32

[0177] In Embodiment 32, the well layers and the barrier layers includedin the emission layer of each of Embodiments 7 and 9 to 11 are changedto Al_(0.1)Ga_(0.9)N_(0.96)As_(0.04) well layers (thickness: 5nm)/Al_(0.01)Ga_(0.99)N_(0.99)As_(0.01) barrier layers (thickness: 5 nm)of two cycles, and further an n-type Al_(0.15)Ga_(0.85)N shielding layeris employed between an n-type light guide layer and the emission layerwithout employing a p-type shielding layer between the emission layerand the p-type light guide layer. Then, an effect similar to that ofeach of Embodiments 7 and 9 to 11 is attained in Embodiment 32 also.

Embodiment 33

[0178] In Embodiment 33, an optical device is prepared by utilizing thenitride semiconductor laser according to any of Embodiments 7 to 10. Inthe optical device utilizing, e.g., a bluish-purple (emission wavelengthof 400 to 410 nm) nitride semiconductor laser according to the presentinvention, the lasing threshold current density is lower as comparedwith a conventional nitride semiconductor laser, the quantity ofspontaneously emitted light included in the laser beam is reduced andthen noise light is reduced. Such a laser device, having a high output(50 mW) and capable of stably operating in a high-temperatureatmosphere, is suitable for a recording/reproducing optical device for ahigh-density recording/reproducing optical disk.

[0179] In FIG. 17, as an exemplary optical device including a laserdevice 1 according to the present invention, an optical disk informationrecording/reproducing apparatus including an optical pickup 2 is shownin a schematic block diagram. This optical informationrecording/reproducing apparatus modulates a laser beam 3 in an opticalmodulator 4 in response to input information and records the informationon a disk 7 through a scanning mirror 5 and a lens 6. A motor 8 rotatesthe disk 7. In reproduction, a detector 10 detects a reflected laserbeam optically modulated by pit arrangement on the disk 7 through a beamsplitter 9, thereby obtaining a reproduction signal. A control circuit11 controls the operation of these elements 1, 4, 5, and 8. The outputof the laser device 1 is generally 30 mW in recording, and about 5 mW inreproduction.

[0180] The laser device according to the present invention is not onlyemployable for the aforementioned optical disk recording/reproducingapparatus, but is also employable for a laser printer, a projector withlasers of the three primary colors (blue, green and red) of light, orthe like.

Embodiment 34

[0181] In Embodiment 34, the nitride semiconductor light-emitting diodeaccording to each of Embodiments 12 and 13 is utilized for an opticaldevice. For example, it is possible to prepare a white light sourceincluding light-emitting diodes or super-luminescent diodes of the threeprimary colors (red, green, blue) of light employing the emission layersaccording to the present invention, and it is also possible to prepare adisplay employing these three primary colors.

[0182] When such a white light source utilizing the light-emittingdevices according to the present invention is employed in place of ahalogen light source employed for a conventional liquid crystal display,it is possible to obtain a backlight having low power consumption andhigh luminance. In other words, the white light source utilizing thelight-emitting devices according to the present invention can be used asa backlight for a liquid crystal display of a man-machine interface in aportable notebook-type computer or a portable telephone, and alsoenables provision of a miniaturized high-definition liquid crystaldisplay.

Industrial Applicability

[0183] According to the present invention as described above, it ispossible to provide a nitride semiconductor light-emitting device havinghigh luminous efficiency and an optical device including the same byintroducing Al into a quantum well layer ofGaN_(1−x-y-z)As_(x)P_(y)Sb_(z) (0<x+y+z≦0.3).

1. A nitride semiconductor light-emitting device comprising an emissionlayer formed on a substrate, said emission layer including a singlequantum well layer of GaN_(1−x−y−z)As_(x)P_(y)Sb_(z) (0<x+y+z≦0 3)containing Al.
 2. The nitride semiconductor light-emitting deviceaccording to claim 1, wherein a content of said Al is at least6×10¹⁸/cm³.
 3. The nitride semiconductor light-emitting device accordingto claim 1, wherein said substrate is a nitride semiconductor substrate.4. The nitride semiconductor light-emitting device according to claim 1,wherein said substrate is a pseudo GaN substrate.
 5. The nitridesemiconductor light-emitting device according to claim 1, wherein athickness of said single quantum well layer is in a range of at least0.4 nm and not more than 20 nm.
 6. The nitride semiconductorlight-emitting device according to claim 1, wherein said single quantumwell layer contains at least one kind of dopant selected from Si, O, S,C, Ge, Zn, Cd and Mg.
 7. The nitride semiconductor light-emitting deviceaccording to claim 6, wherein a content of said dopant is in a range of1×10¹⁶/cm³ to 1×10²⁰/cm³.
 8. The nitride semiconductor light-emittingdevice according to claim 3, wherein the etch pit density of saidsubstrate is not more than 7×10⁷/cm².
 9. An optical device utilizing thenitride semiconductor light-emitting device of claim
 1. 10. A nitridesemiconductor light-emitting device comprising an emission layer havinga multiple quantum well structure obtained by alternately stacking aplurality of quantum well layers and a plurality of barrier layers, saidquantum well layers being formed with GaN_(1−x−y−z)As_(x)P_(y)Sb_(z)(0≦x≦0.10, 0≦y≦0.16, 0≦z≦0.04, x+y+z>0) and additionally containing atleast Al, said barrier layers being formed with a nitride semiconductor.11. The nitride semiconductor light-emitting device according to claim10, wherein the Al content in said well layers is at least 1×10¹⁹/cm³.12. The nitride semiconductor light-emitting device according to claim10, wherein said barrier layers further contain any element selectedfrom As, P and Sb.
 13. The nitride semiconductor light-emitting deviceaccording to claim 10, wherein said emission layer includes at least 2and not more than 10 said well layers.
 14. The nitride semiconductorlight-emitting device according to claim 10, wherein said well layershave a thickness of at least 0.4 nm and not more than 20 nm.
 15. Thenitride semiconductor light-emitting device according to claim 10,wherein said barrier layers have a thickness of at least 1 nm and notmore than 20 nm.
 16. The nitride semiconductor light-emitting deviceaccording to claim 10, wherein at least one kind of dopant selected fromSi, O, S, C, Ge, Zn, Cd and Mg is added to at least either said welllayers or said barrier layers.
 17. The nitride semiconductorlight-emitting device according to claim 16, wherein a content of saiddopant is in a range of 1×10¹⁶ to 1×10²⁰/cm³.
 18. The nitridesemiconductor light-emitting device according to claim 10, including asubstrate for growing a plurality of semiconductor layers included insaid nitride semiconductor light-emitting device, wherein at leasteither a first adjacent semiconductor layer in contact with a first mainsurface, included in both main surfaces of said emission layer, closerto said substrate or a second adjacent semiconductor layer in contactwith a second main surface farther from said substrate is formed with anitride semiconductor containing Al.
 19. The nitride semiconductorlight-emitting device according to claim 10, wherein said light-emittingdiode is formed using a GaN substrate.
 20. An optical device includingsaid nitride semiconductor light-emitting device of claim 10.